The invention relates in general to stretch blow mold machinery. More particularly, the invention relates to an improved stretch blow mold shell, and a shell holder assembly therefor in which the shell holder is constructed and arranged to be mounted to one of a pair of pivoting clamp brackets used to pivotably close an opposed pair of the mold shells on themselves for use in defining a bottle cavity during usage thereof on a stretch blow mold machine.
The use of stretch blow mold machinery is well known. Stretch blow mold machines offer the advantage of quickly and economically producing a wide variety of bottle shapes and sizes commonly used throughout a great number of consumer markets. These include longneck beverage bottles, wide mouth bottles, as well as a wide variety of soft drink and juice bottles, for example, ranging in size from approximately half liter up to 5 liters.
The use of rotating stretch blow mold machines constructed to have a rotating central frame member, or carousel, which rotates about a fixed or stationary central frame member is well known. A plurality of radially spaced bottle forming stations are typically stationed along the circumference of the carousel, each bottle forming station comprising a pair of opposed and pivotably affixed clamp brackets constructed to move from an open into a closed position, and back, in timed relationship with the rotation of the framework during machine operation.
In the known machines, a pair of shell holders, which may be identical or may differ from one another, but which, however, are substantially uniform in size and shape with respect to one another will be separately received within each respective one of the clamp brackets pivotably affixed to the blow mold machine carousel. These shell holders are fastened to the clamp brackets by a plurality of threaded fasteners, and may also be provided with cooling channels defined therein so that cooling water may be passed therethrough in the effort to regulate the interior surface temperature of a separate one of the blow mold shells fastened thereto and carried thereon, and which are used to define the bottle forming cavity which will form the bottle, or other object, during usage of the machine.
Each blow mold station thus also includes a pair of blow mold shell halves. These blow mold shell halves are typically identical, such that a symmetrical bottle, or other article, will be formed during the stretch blow mold process. Each mold shell is fastened to a respective one of the shell holders such that the completed bottle forming station will have a pair of opposed clamp brackets which are pivotably affixed to one another, much in the fashion as disclosed in U.S. Pat. No. 5,326,250 entitled xe2x80x9cOpening and Closing Mechanism for Portfolio Blowing and Blowing Stretching Moldxe2x80x9d issued Jul. 5, 1994 in the name of inventor Doudement, and as well as in U.S. Pat. No. 5,683,729 entitled xe2x80x9cApparatus for Making Containers by Blow Moulding Plastic Parisonsxe2x80x9d issued to Nov. 11, 1997 in the name of inventor Valles; a shell holder fastened to each respective clamp bracket; and in turn a mold shell half affixed to the shell holder by suitable means. Common methods of attaching these shell halves to the shell holders include the use of predefined cavities formed within the shell holder within which the shell is placed and then held in position by a keeper plate or plates fastened to the shell holder, or by passing fasteners directly through the shell and into the shell holder.
Current rotating stretch blow mold machines are capable of producing up to 50,000 articles per hour, based on the size of the articles being produced, for example half liter bottles as opposed to five liter bottles. However, an advantageous feature of certain stretch blow mold machines, and particularly those manufactured by Sidel, S.A. of Le Havre, France, and its U.S. subsidiary Sidel, Inc., is that through the use of a modular design, for example modular mold shell halves, and modular shell holders, it is possible to use one rotating blow mold machine in conjunction with several sets of shell holders, and mold shells, to produce a wide variety of bottles, for example, using only one machine. This greatly reduces machine costs, and increases operating efficiencies for stretch blow mold bottle producers. However, the change-over from molding bottles of one size to bottles of another size can be quite time consuming due to the amount of time required to remove and replace the shell holders from each pair of clamp brackets about the periphery of the machine, as well as removing and replacing the shells from the respective shell holders.
Another disadvantage of the known types of blow mold shells and shell holders is the manner in which the blow mold shell is cooled during the bottle formation process. As known to those of skill in the art, when using a PET (polyethylene terephthalate) preform, also known as a parison, it is desirable to maintain the temperature of the neck and/or threaded neck portion of the bottle at a cooler temperature than the remaining portion of the preform during the stretch blow mold process such that a sufficient amount of structural strength and rigidity is provided at the neck so that it may be used to safely convey the bottle along a packaging line, for example, whereupon the bottle is filled and then sealed/capped, and carried at its neck for being packaged and shipped. Thus, it is desirable to maintain a thicker wall section at the neck portion of the bottle, as well as at the base portion, for the purposes of lending structural rigidity to the bottle.
A cooling circuit, therefore, may be integrally defined within the neck or upper portion of the shell halves to cool the neck portion of the PET container as it is being formed, and which may also be used to cool the sidewall portion of the shell, although there is only one cooling circuit in these known constructions. The wall section of the bottle, on the other hand, which extends between the neck and base, will typically be molded in a thinner section between the neck and base in that as the bottle is empty the sidewall does not need to have a great deal of strength. Once the bottle is formed, all that is required is a sufficient amount of strength along the sidewall of the bottle to ensure that the bottle does not collapse while being handled, nor rupture once it is filled with the appropriate fluid. As known to those in the art, once the fluid is constrained within the bottle, the fluid itself acts as a constrained load carrying column which will support its own weight, so long as sufficient sidewall strength exists within the container sidewall to ensure it will not rupture.
Thus, it is desirable to mold bottles and other articles with as thin a wall section as possible, as the cost of the plastic PET material is the largest part of the expense in making bottles in the stretch blow mold process. As it is desirable to reduce the per unit costs of each bottle produced, it is desirable to precisely and separately control the temperature of the sidewall of the preform/shell as the bottle is formed to be able to provide the minimal amount of PET material required to form the bottle by keeping the sidewall warmer/hotter than the neck or base portions, for example.
In order to attempt to control the temperature of the sidewall portion of the shell/bottle, a cooling channel is drilled or otherwise defined within the shell holder, which channel will extend in the lengthwise direction of the holder and be positioned as closely as possible to the sidewall of the mold shell in order to try to cool the interior surface of the mold shell during the stretch blow mold process. Although this type of construction has proven to be adequate for high volume strength blow mold bottle production, the problem persists that a thicker wall section is formed because of the inability to separately control the temperature of the sidewall portion of the mold shell with respect to the neck portion thereof, which thus unnecessarily drives up per unit costs.
Another problem with the known types of blow mold shells and shell holders as used with rotating blow mold machines relates to the mass of the shell holder when situated in the clamp brackets, all of this being carried along the circumference of a rotating device, such that the inertial effect of this weight must be dealt with, and overcome, in order to get this mass moving before and during the bottle forming process at the desired operating speed. Due to the high speed with which rotating blow mold machines operate, it is desirable, therefore, to reduce the mass of the shell holder as well as reducing the mass of the shell in order to reduce this inertial affect, and to allow for still greater operating speeds, and which will also increase machine service life by reducing the jarring of the machine from this inertia/mass problem.
Lastly, once the size of the mold shell and the shell holders has been reduced, for the reasons described above, the problem of mold shell xe2x80x9ccompensationxe2x80x9d arises. In the known constructions of blow mold shells and shell holders, an air injection circuit is used to inject air into the PET preform during the stretch blow mold process. This creates an internal air pressure within the shell that tends to separate the shells from one another as the bottle is being formed, the bottle acting as a large catheter or balloon that seeks to move the shells apart from one another. To combat this, an equal amount of air pressure has been introduced into the known types of shell holders and against the clamp brackets to equalize or xe2x80x9ccompensatexe2x80x9d for this air pressure such that the mold shells will stay closed for forming a bottle without an unsightly or undesired sidewall seam along the length thereof This has been done with a common air circuit providing air to the bottle forming cavity, and the compensation area. However, this has typically required a 1:1 equalization of the air pressure/surface area of the shell holder/clamp bracket compensation area and the bottle forming cavity surface area within the shells in order to compensate accordingly. However, when reducing the size of the shells, as is done in the invention disclosed herein, a sufficient amount of surface area is not available to provide 1:1 surface/air pressure equalization between the shell holder and shell.
Therefore, a need exists for an improved blow mold shell, shell holder, and clamping bracket assembly which is constructed and arranged to allow for the quick change-over of the shells and shell holders such that machine downtime during change-over is minimized; which allows for separate internal cooling of the sidewall of the shell, directly, without having to first go through a shell holder; which reduces the mass of the shell holder and shell arrangement to offer greater machine operating speeds; and which will allow for pressure compensation of the shell/shell holder/clamp bracket during the blow mold injection process such that the mold shell halves remain closed on one another during the bottle molding process.
Accordingly the present invention provides for a blow mold assembly that benefits from increased productivity and decreases in material costs. By providing for a blow mold shell that is capable of accurately controlling the molding temperature in the various regions of the mold cavity, wall thickness of the containers can be precisely controlled, thus, optimizing the amount of preform material used to fabricate a specific container. Additionally, by providing for a lightweight mold holder design the present invention increases overall blow-molding throughput by decreasing the time required to fabricate a specified container. The streamlined blow mold assembly of the present invention also benefits from ease in changeover of mold shells and mold holders during mold fabrication due in part to the light-weight, simplistic design. Overall, the blow mold assembly of the present invention provides for an efficient and cost-saving means for manufacturing blow-molded containers.
In one embodiment of the invention the blow mold assembly that is used to manufacture blow molded containers comprises a blow mold shell, a mold holder and a mold holding assembly. The blow mold shell has a first annular groove disposed within the lengthwise exterior surface of the shell. The first annular groove serves as a mating surface for a support key that interlocks the mold shell within the blow mold assembly. The unique placement of the annular groove, proximate to the end of the blow mold shell nearest the neck portion of the shell""s cavity allows for two or more independent cooling channels to exist within the mold shell. By providing for a first cooling channel capable of cooling the sidewall portion of the molded container and a second cooling channel capable of independently cooling the neck portion of the molded container, wall thickness can be optimized in specified areas of the container to minimize the amount of material required to fabricate a structurally sound container. Alternatively, the second cooling channel or an additional cooling channel may be located within a separate shoulder insert placed within the interior surface of the blow mold proximate the neck portion of the mold.
Additionally, the blow mold assembly comprises a mold holder that includes arcuate interior surfaces and arcuate exterior surfaces so as to create a concentric blow mold holder. This construct of a blow mold holder benefits from less mass, thereby significantly increasing production rates and overall ease during time consuming changeover periods for mold shells and shell holders. The mold holder may comprise a pressure compensation element that is capable of compensating for the pressure build-up within the mold shell during the blow mold process.
The blow mold assembly also comprises a blow molding holding assembly that comprises clamping brackets having arcuate inner surfaces for mating with the arcuate exterior surfaces of the mold holder or mold shell. The clamping brackets are pivotally driven by a cam apparatus within a blow-molding machine and serve to open and close the blow mold assembly as warranted by the fabrication process. The quick release nature of the clamping brackets allow for mold shells to be removed and inserted with greater ease during the fabrication process thus, increasing the efficiency of the overall blow mold assembly.
In another embodiment of the invention the blow mold shell assembly comprises a mold shell and a mold holding assembly. This assembly is consistent with the above-discussed assembly except for the omission of the mold holder. While most mold shell assemblies will implement a mold holder for ease in mold shell changeout, it is possible to construct the blow mold assembly of the present invention without the need to include a mold holder. In such a relationship, the mold holding assembly comprises clamping brackets having an arcuate inner surface and the arcuate inner surface of the clamping brackets are disposed circumferentially about the exterior surface of the blow mold shell.
In other embodiments of the invention the individual blow mold shell, shell holder and blow mold holding assembly are defined per the previous discussion.
As such, the blow mold assemblies that have these improved performance characteristics, and many others that will be readily apparent to those of ordinary skill in the art, are desired for improving the overall cost efficiency and production throughput of blow mold manufactured containers.